The present invention relates to apparatus for optically reading printed information such as bar codes, letter or numerical characters or the like printed on a base such as a label or the like.
In a conventional reading apparatus of this kind such as disclosed in Japanese laid-open patent application No. 56-33770 published Apr. 4, 1981, a tungsten bulb has been used to radiate the light onto information such as bar codes, numerical or letter characters or the like printed on a white base so that reflected light corresponding to the printed information is provided, and an electronically scanned image sensor has been used to convert the information image of the reflected light into a corresponding electric signal.
When the information is printed in black using black ink on a white base, the reflected light is very contrasting because of high and low light reflectivities of the white portion and the black portion, respectively, over a whole range of wavelengths of the illuminating light from a tungsten bulb. Thus, no disability in information reading arises.
However, when the information is printed in black using a thermal printer on a heat sensitive white base, the reflected light becomes less contrasting as the wavelength of the illuminating light becomes longer because of the increased light reflectivity of the black portion in the longer wavelengths of the light. Since the illuminating light from a tungsten bulb has many infrared light wavelengths which are longer than the wavelengths of visible light, the reflected light is not contrasting enough to be read and an information reading disability is encountered.
For this reason, an infrared light filter which cuts off the infrared wavelength component has usually been provided to lower the light sensitivity in the infrared light wavelength range in view of the fact that the image sensor has a high light sensitivity in the range of the infrared light wavelengths.
However, even if an infrared light cut-off filter is used, the cut-off characteristic of the filter is not nullified in the range of the visible light wavelengths in spite of the considerable decrease in the wavelength range from the near infrared light to the visible light. Therefore, the light radiation intensity of a tungsten bulb must be increased to compensate for the decrease in the spectral sensitivity due to the filter operation. This results in a large-sized light source and an increase in the generated heat issuing from the light source.